Additive Manufacturing with Reprap Methodology: Current Situation and Future Prospects

Additive manufacturing with RepRap methodology: current situation and future prospects L. Romero1, A. Guerrero1, M. M. Espinosa1, M. Jiménez 2, I.A. Domínguez1, M. Domínguez1 1 Design Engineering Area - Universidad Nacional de Educación a Distancia (UNED), Madrid, Spain 2 Department of Mechanical Engineering, Technical School of Engineering - ICAI, Comillas University, Madrid, Spain Abstract In February 2004, Adrian Bowyer, from the University of Bath, began an open initiative called RepRap, with the purpose of creating an open source rapid prototyping machine which, moreover, could replicate itself. This article analizes the current status of the RepRap initiative, commenting the basic components of RepRap machines, the differences between the different 3D printers developed by the RepRap community so far, and the technical possibilities that opens this technology from the engineering point of view. In addition we propose some improvements that could be perfectly feasible in the short term. For this purpose, the assembly of a RepRap Mendel Prusa was performed, but with some modifications. 120 1. Introduction to RepRap community. A 3D printing machine (and any computing device in general) is composed of hardware (electric, electronic, electromechanic and mechanic physical components) and control software (the logical equipment as the operating system or software applications), usually both developed by companies that control them. However, there are several companies in the world that are dedicated to the development of hardware in a peculiar way: they publish the sources of their inventions and sharing them with the whole world, with the aim that users from all the planet can use them and propose improvements. This is the model of open source hardware: hardware whose design, components, tools and documentation are open and they are publicly available to anyone. The origin of these companies is varied: some were linked to projects without profit; others employ directly the open source model; in other cases focus on the research and some educational projects (such as the case of Arduino or RaspberryPi). These initiatives, by the fact of being open, have made possible the emergence of other companies that have been able to develop commercially an original product or offer services and products around that initial product. As in other cases related to free culture, this economy of open hardware is based on the quality of the product and the company innovation capacity and not in the sale of single copies, although it may be under a trademark. This model seems to work: companies like Sparkfun, Adfruit, Ultimaker, Cooking Hacks or RepRap BCN produce open hardware. RepRap is a rapid prototyping machine, modeled by melting and solidification with a nozzle, self-replicating, and with open source. RepRap is being developed by an open community on RepRap.org, which was founded in 2004 by Adrian Bowyer at the University of Bath. As the term Fused Deposition Modeling is registered by Stratasys, the RepRap community has coined the term Fused Filament Fabrication (FFF) that can be used by anyone without restriction [1]. RepRap is self-replicating because it is capable of generating a significant fraction of its own components. RepRap is registered by the University of Bath, and is open source because it is registered under a 2 GPL versión [2]. Under these conditions, any person can distribute and amend a RepRap machine, but must maintain the modifications under this license. The changes must continue to be publics. As the machine is free and open source, anyone can build, without the payment of any fee, an unlimited number of copies, for him or any other, using RepRap machines to build the plastic parts of the copies. The founders of RepRap related the prototyping machine with a biological metaphor. They compared it with a mutualism relationship, also known as symbiosis. In this relationship, the machine benefits from the human through its support for playback: the human has to complete the assembly of the new machine with the parts that are not self- replicating and assemble the machine. And the benefit of the human is the use that makes from the machine in order to create the prototypes that he wants. Due to the biological inspiration of their creation, RepRap machines have been baptized with surnames of well-known biologists (Charles Darwin, Gregor Mendel, Thomas Henry Huxley and Alfred Russel Wallace). 121 2. RepRap models and features. 3D printers developed by RepRap community have the following characteristics: They are machines of the type FFF (Fused Filament Fabrication), and they use thermoplastic compounds (usually ABS and PLA). Four models have mainly been developed to date (Darwin, Mendel, Huxley and Wallace) with different variants (Prusa Mendel, MendelMax, RepRapPro Huxley, RepRapPro Mendel, etc). The accuracy of the model is given by the diameter of the nozzle, which is usually between 0,4 - 0,5 millimeters. Darwin was the first model developed by RepRap in 2007 (Figure 1). This first version is currently obsolete. Figure 1: RepRap Darwin. Mendel is the second model developed by the RepRap community, and it is the most popular to date, at least in terms of number of variants. The support structure is a triangular pyramid, formed by M8 threaded rods and printed pieces (Figure 2). The most widespread variant is the RepRap Mendel Prusa, created by Josef Prusa. Figure 2: RepRap Mendel. 122 There is a Mendel variant with the pyramidal structure formed by aluminum profiles (MendelMax) which was created by Maxbots in December 2011. These profiles provide easer assembly and greater rigidity, with a small increase in price. A variant with three extruders has also been developed: the RepRap Tricolour Mendel which was created by RepRapPro Ltd in 2013. The extruders use Bowden tubes which reduce the total weight to support. Huxley is the third model and it was created by RepRapPro (Figure 3). It is the small and transportable variant of the Mendel Prusa, using M6 rods and NEMA 14 motors, like the RepRap Wallace. Figure 3: RepRapPro Huxley. Wallace is the smallest model of the RepRap family. It is designed with M6 threaded rods and NEMA 14 motors. It is the most simple and easy to assemble variant (Figure 4). It is an evolution of the Printerbot, the simplest and cheapest RepRap printer. Figure 4: RepRapPro Wallace. 123 3. Basic components of RepRap machines. The basic components of RepRap machines can be classified according to the type of components or the function that they execute. a) Components by type. There are four groups in this category: Plastics components: They are replicable components. They can be manufactured by a RepRap. In Darwin and Mendel machines, the plastic components represent 48% of the total of the elements [3]. The material that forms them is normally PLA (polylactic acid) or ABS (acrylonitrile butadiene styrene). They include structural components, extruder components, carriage print components, etc. Figure 5 includes a picture of a complete set of printed parts. The printer has been designed in order to use metal bearings which are easy to find: circular metal bearings are used by skates’ manufacturers and they are sold as spares in sport equipment stores Figure 5: Complete set of printed parts Non-replicable structural components ("vitamins"): includes threaded and smooth bars, washers, nuts, circular and linear bearings, special parts for the extruder, belts... They are usually made of metal, steel or aluminum, except the belts that are usually plastics or flexible rubber. Electronic components (“more vitamins"): they are the necessary electronic components for the printer operation, like microcomputer and electronic board, stepper motors, temperature and end stop sensors, resistances and cables. The movement of the machine axes is achieved by stepping motors. The axis of these motors takes a turn with a fixed angle whenever it receives an electrical impulse. The motors that are often used are the NEMA 17, which is a standard of the National Electric Manufacturers Association (NEMA). Software: The RepRap microcomputer (firmware) and the external computer (for sectioning parts and calculating the GCODE), which are necessary for the equipment operation. 124 b) Components by function. From a functional point of view, the components of RepRap printers can be divided as follows: Support structure: is the structure that supports the whole body of the RepRap. Y, X, Z axes. Y axis movement is forwards/backwards movement; X axis movement is side to side, left to right; and Z axis movement is up and down along the vertical plane. Linear movement is generally accomplished using one of 2 different methods: Belt/pulley driven motion and threaded rod, or lead screw motion. The extruder: is responsible for feeding filament through a nozzle and melting it as it's deposited into the bed where the part is made [4]. The extruder (Figure 6) consists of two parts: The cold end and the hot end. Figure 6: Extruder and extruder parts made by a FFF machine. The print bed: is what the RepRap extrudes plastic into, where the plastic parts are built up. The print bed may be stationary, like the original reprap Darwin, or it may move along one of the x/y/z axes. The bed usually consists of two plates: the upper plate and the lower plate. The heated bed. While a heated bed is considered an optional component of a RepRap, it often becomes necessary for operating a RepRap long-term because, without a heated bed, parts have a tendency to cool down too quickly. The fusor: melts the fiber and puts it through the nozzle. The end stop: is a very small and simple circuit board with a switch that tells the reprap when it has moved too far in one direction. Thus, there's normally 6 of these: 2 for each axis.

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